Field
This disclosure relates to the making of ophthalmic elements and in particular to methods of improving the quality of eyewear lenses made by using additive techniques, and the products obtained therefrom.
Description of the Related Art
Prescription eyewear lenses (spectacle lenses or ophthalmic lenses) are used to correct refractive errors of human vision. Eyewear lenses may be used for protection, namely to protect the eyes from excess light or from a mechanical, chemical or biological hazard.
Two main technologies are used to shape the surfaces of eyewear lenses. One shaping technology is molding. With molding, a reference surface is made in glass or other material, and the reference surface is replicated on the eyewear lens by mold casting or injection molding. The other shaping technology is grinding and polishing, where a solid piece of optical material is mechanically ground (or milled) and polished until the required surface is obtained.
Prescription eyewear lenses are characterized by the large number of possible different lenses that can be potentially made. Lenses can be different in regard to lens material, modifiers embedded in the lens, coatings, power and power distribution. As refractive errors are measured in steps of 0.25 D, which is considered to be the distinguishable average threshold for human blur perception, there is a limit to the number of different optical powers that could be potentially used. Nevertheless, modern digital lenses available since around 2000 may be tailored to the user's characteristics or needs, taking into account the actual position of the lens with respect to the eye. As a consequence, the optimization of the optical performance of the eyewear lens for each individual requires the capacity of producing lenses with a large variety of optical powers. Similarly, multifocal eyewear lenses used primarily for presbyope individuals have a continuous variation of power across the lens aperture, allowing the user to sharply focus at different distances. Once again, the number of different power distributions in these multifocal lenses is virtually infinite.
Modern digital grinding (milling) and polishing technology may be used to produce arbitrary surfaces. In this way it is well suited for making eyewear lenses with the required shape variability. However, mechanical grinding and polishing is a subtractive process that requires complex and expensive machinery to guarantee the accuracy level required for eyewear products. Mechanical grinding and polishing requires expensive tooling and equipment for proper alignment, and uses expensive consumables including cutting tools, polishing slurries, polishing pads, and coolants. Plus, mechanical grinding and polishing is an energy consuming process that also produces a lot of waste, which is difficult to handle and eliminate.
Molding is not a subtractive method, but it requires the previous manufacturing of the mold itself. When the number of required lens surfaces is too numerous, molding becomes unpractical because the number of lens surfaces molded (or cast) from each mold are too low in number, resulting in a large number of molds that must be produced and kept on hand.
The optical industry has employed a hybrid method combining molding and mechanical techniques, which is possible thanks to the additive nature of optical power. A lens element has two polished surfaces, and its optical power can be distributed between the two surfaces. One of the surfaces can be normalized to a relatively small number of different shapes, which are made by molding, and the other surface is ground/polished to a shape required to obtain the desired optical power and optical power distribution. This way, the manufacturing of eyewear lenses is split in a two-step process. First, semi-finished lens blanks are produced by casting or injection molding. A lens manufacturer buys and stores these blanks, or produces the blanks. Second, when a lens manufacturer receives a lens order, an appropriate lens blank is selected and the back surface of the blank is mechanically figured to create the requested lens.
The customized nature of eyewear lenses makes them ideal products for a more distributed and simple manufacturing process. An improved additive process for eyewear lens preparation would be beneficial. The lenses could be made on demand which has multiple benefits. Custom manufacturing lenses using an additive process removes the need to produce and store semi-finished blanks, eliminates material waste inherent in grinding, and reduces energy consumption by simplifying the overall process. Also, additive manufacturing allows for adding or embedding structures for upcoming technologies inside the lens including using mirrors, prisms, micro-lenses, diffraction gratings, light sources, and other techniques.
Throughout this description, elements appearing in figures are assigned three-digit reference designators, where the most significant digit is the figure number and the two least significant digits are specific to the element. An element that is not described in conjunction with a figure may be presumed to have the same characteristics and function as a previously-described element having a reference designator with the same least significant digits.